cosmology - Is it valid to apply Einstein's Relativity to scenarios involving expansion of space?

IIs it valid to apply Einstein's Relativity to scenarios involving expansion of space? For a practical example of this: Is it legitimate to speak of distant red-shift galaxies as experiencing time more slowly in relation to our experience of time? I appreciate that isn't sensible in other ways, but by explaining if it is legitimate if albeit not-sensibly, you are kindly helping me understand a bit more about Relativity and expansion of space :O)